Magnesium oxychloride (MOC) cement, also known as Sorel cement, has been used in many parts of the world. It is made by lightly burned magnesium oxide, magnesium chloride, water, and additives. It is commonly believed that the 5-1-8 phase, where the MgO:MgCl2.6H2O:H2O molar ratio is 5:1:8, is the desirable phase which contributes the most strength. Research indicated that ideal ratios for the raw materials vary between 6:1:13 to 9:1:17 for optimum strength, dimensional stability, and water resistance.
MOC is a non-hydraulic cement that's light, strong, fire retardant, mold resistant, and easy to cut and fasten. Comparing to Ordinary Portland cement, MOC is less water resistant and potentially poses fastener corrosion problems due to the presence of high levels of chloride.
Magnesium oxysulfate (MOS) and magnesium phosphate cements are other members of the magnesia cement family. The oxysulfate and phosphate cements are not as widely used as MOC.
Unfortunately, there are many disadvantages to the current MOC boards used in the construction industry. Most of MOC cement is made in Asia in the form of boards reinforced with woven, non-woven, or chopped glass fiber. Typical sizes of the board are 4′×8′ of various thickness, from 3 mm to 50 mm. These MOC boards, known simply as MgO boards on the market place, are used as wall boards, floors, tile backer boards, and other applications. Factors limiting the wide spread use of the MOC boards include high cost, relatively high density, and the perception of poor handling by the construction industry. MOC boards do not handle as the common gypsum drywall boards. Common MOC boards are cut using either carbide or diamond blades and create a lot of dust. For fastening, special counter-sink crews are used to prevent cracking. The boards according to this application can be cut and fastened like regular gypsum boards.
Almost all MOC boards produced today use wood particles as a main filler material. Indeed, almost all MOC boards made today contain 5-20% wood particles by weight. Wood particle fillers have the advantage of very low cost and they improve the flexibility of the finished board. These boards, however, come with many disadvantages. Wood fillers absorb large amounts of water and a formulator has to compromise the performance of the product. The presence of wood also significantly reduce the fire resistance of the MOC boards which they comprise. Being organic in nature, wood particles also support the growth of mold, fungi, and other organisms.
MOC is a sacrificial type fire retardant. When a cured MOC material is exposed to fire, the heat drives a series of reactions where the crystalline water is evaporated. These reactions are highly endothermic and produce a cooling effect. One of the MOC decomposition products is MgO, which is a refractory material that can reflect heat. As a result, MOC loses almost all of its mechanical strength when decomposed by fire. This is an inherent disadvantage for most materials based on magnesia cement.
Further, all current commercial MOC boards have densities >0.8 grams/cm3. For a ½″ thick 4′×8′ board, the weight is more than 65 lbs. This is considered as too heavy by the construction industry. Prior art technologies have attempted to solve this weight problem through the use of expanded polystyrene (EPS) beads as fillers. EPS has the advantage of very low density, low cost, and low water absorption. However, EPS is a low melting organic material that significantly reduces the fire performance of the MOC material. EPS also significantly reduces the physical strength of MOC. Similar problems also arise when low density boards are made through the use of foamed MOC cement. These boards have reduced fire retardancy and low strength.
Other attempts to resolve these issues may be found in Chinese patent CN1098394A (1995), which disclosed MOC formulations and additives to overcome the drawbacks of MOC cement such as hygroscopicity, efflorescence, and dimensional instability. The minimum MgCl2 concentration in the Chinese patent was 18 Baume, which is higher than what would be needed for industrial practicability. In addition, no surface treated fillers were used and low density was not mentioned. Similarly, US2014/0079942 disclosed coating compositions containing essentially magnesium oxide, potassium phosphate, and calcium phosphate. Phosphate, however, is very expensive and is believed to be not suitable for construction boards. Further, boards coated with magnesium phosphate cement coating are inferior to MOC based products in terms of fire performance. U.S. Pat. No. 7,255,907 and US20090065972 disclosed MOC board compositions and procedures. The claims in the '907 and '972 patents are similar to commercial boards on the market. Such boards, however, generally have a density of >0.9 grams/cm3. U.S. Pat. No. 8,066,812 disclosed formulations with densities >1.0 grams/cm3. When these materials are used as wall board, a ½″ thick 4′×8′ board weighs more than 70 lbs. This is too heavy to be reasonably used in the construction industry. In addition, the current materials disclosed in this art are difficult to paint or join with joining compounds.
There is, therefore, a need for low density, low cost, high performance MOC building materials. There is further a need for such MOC building materials to be water resistant, retain high strength after exposure to fire, and be able to reduce corrosion.